The present invention relates generally to semiconductor devices and more particularly to a semiconductor device having a low thermal coefficient.
It is well known in the prior art to form a zener diode at the base of an NPN bipolar transistor. An extra N+ diffusion into the base region provides a simple method of forming the reverse biased zener diode in series with the base. As illustrated in FIG. 1, the integrated circuit of the prior art generally includes an N substrate collector 10, a diffused P base region 12, an N+ emitter region 14 formed in the base and an N+ cathode 16 also formed in the base. Metal contacts are shown as collector contact 18, base contact ring 20 surrounding emitter contact 22 and a zener cathode contact 24. This structure, which is disclosed in the U.S. Pat. No. 3,275,846, has a low thermal coefficient but is still susceptible to operating characteristics deviations with temperature changes. The purpose for forming the zener diode in the same island or region of the base emitter of an NPN bipolar transistor is to achieve some form of stability of operating characteristics with changes in temperatures. The design concept is based upon the fact that the reverse biased zener diode has a positive voltage temperature coefficient and the forward biased base emitter PN junction has a negative voltage temperature coefficient. Thus by locating the zener diode and the base-emitter diode as close as possible, it would be affected by the same temperature variance and produce some form of compensation. This relationship is specifically described in U.S. Pat. No. 3,567,965.
Although the device in FIG. 1 has provided an improvement over the prior art devices wherein the zener diode is formed in a separate distinct region from the NPN bipolar transistor, it still does not provide sufficient tracking of the variance of the zener diode characteristics with the base-emitter diode since the base-emitter junction is not symmetric around the breakdown region. Additionally, the zener diode is of the surface type which has high noise and poor stability. Also for high Beta integrated circuit transistors, there is a high probability for the prior art integrated circuit of FIG. 1 of a depletion region spreading into the P base region 12 from the N+ cathode 16 and reaching the collector region 10 before the zener breakdown is reached. If this phenomenon which is called "reach through," occurs, current flows from the cathode of the zener diode into the collector and thereby defeating the intended operation of the component.
Thus their exists a need for an improved integrated circuit having reverse biased zener diode in combination with a base-emitter forward biased diode having improved thermal characteristics.